Computer systems and other electronic systems typically include a user interface enabling a user to interact with and control the systems. For example, a computer system often includes some type of monitor to display information, a keyboard with which to type input and a mouse or other pointing device to position a cursor within a graphical user interface. For portable devices and for computer systems and other electronic systems in public places or with other special requirements, these typical user interfaces may be supplemented with or replaced by a touch sensitive screen. For example, medical monitoring systems in a hospital or industrial process control systems in a factory may be much more convenient to operate by touching a touch sensitive display screen rather than using a keyboard and mouse. Touch sensitive screens are also particularly beneficial for portable devices with limited space for input devices, such as cellular telephones, personal digital assistant (PDA) devices, and portable electronic game systems. A touch sensitive screen or touch screen system displays information such as text, graphics, or other virtual or software control devices (switch, button, scroll bar, dial and so on), and detects the location of a touch on the screen.
A number of different types of touch screens are known using a variety of touch sensing technologies, such as resistive, capacitive and surface acoustic wave (SAW) systems. A resistive touch screen use a touch sensitive overlay over a display. The overlay has two layered transparent sheets separated by insulating spacer dots. The inner surfaces of the two sheets are coated with an electrically conductive coating, and a wire is attached to each edge of each sheet. When the touch screen is pressed, the two sheets are pressed into contact and an electrical circuit is completed through the two sheets. The resistance between a wire at the edge of one sheet to another wire at the edge of the second sheet can be used to determine the location of the touch. However, a resistive touch screen can detect the location of only one touch at a time. If the resistive touch screen is simultaneously pressed in multiple locations, the location will not be correctly determined.
A capacitive touch screen includes a capacitive storage layer placed over a display. When a user touches the capacitive touch screen, some of the electrical charge on the capacitive storage layer is transferred to the users finger. Detectors at the four corners of the capacitive storage layer are used to determine the distance to the contact based on the relative differences in charge at the corners. However, a capacitive touch screen is more expensive and complex than a resistive touch screen, is susceptible to electrical noise and cannot be used with a stylus or gloved finger. A surface acoustic wave (SAW) touch screen uses acoustic transducers and reflectors on the glass surface of a display, detecting where an acoustic wave is disturbed by a touch on the glass surface. The SAW touch screen is even more expensive than a capacitive touch screen, has a higher power consumption than a resistive touch screen and may be sensitive to environmental conditions such as surrounding objects, dirt or moisture.
Portable electronic devices such as cellular telephones, personal digital assistant (PDA) devices, and portable electronic game systems be greatly enhanced by a touch sensitive display that can detect multiple simultaneous touches. However, the cost and complexity of existing systems for detecting multiple simultaneous touches on the screen are often prohibitive for these types of portable electronic devices and make their use unattractive even in more expensive electronic equipment.